Temperature sensing module

ABSTRACT

The present invention concerns a sensor module with a hollow mirror (3) at whose focal point a sensor element (4) has been arranged whose output signal is compared with a reference signal and is transformed into a temperature signal in an evaluation circuit (15). The sensor module has a thermopile (6) in whose immediate vicinity a temperature reference element (5) has been arranged; a first pre-amplifier (8, 9), that is capable of being calibrated, amplifies the output signal from the thermopile (6); a second pre-amplifier (10-13) amplifies the output signal from the temperature reference element (5); and a third pre-amplifier (14) is connected into the circuit in the form of a difference amplifier and forms the difference in signal between the outputs from the first pre-amplifier (8, 9) and the second pre-amplifier (10-13).

BACKGROUND OF THE INVENTION

The present invention concerns a temperature-compensated sensor module,especially one for registering infrared radiation, in order to measurethe temperature in domestic electrical devices. Such a sensor module ofthe prior art has a hollow mirror with a focal point at which is locateda sensor element of which an output signal is compared with a referencesignal and is transformed into a temperature signal in an evaluationcircuit, such a sensor module being shown, by way of example, in DE-3843 947 C1.

The domestic device that is described there is a toaster that has beenequipped with an infrared detector whose output signal is compared witha target value via a comparator. The device is switched off in the eventthat the target value is exceeded. The measurement accuracy of thisarrangement varies markedly with the ambient temperature that prevailsat the location of usage of the infrared detector. In order to increasethe measurement accuracy, the infrared detector would have to bethermally insulated from the ambient temperature variations and this istechnically expensive and time-consuming.

A heating/boiling device is known from EP-15 710 D1, especially amicrowave oven, with a sensor module that preferably has a pyroelectricIR detector onto which infrared radiation, that emanates from thearticle that is being cooked (i.e. the object that is being measured),is imaged via an input window--that is termed a "peep hole"--a chopperand a hollow mirror and then through a cylindrical measurement tube witha length of approximately 150 mm and a parabolic mirror that is arrangedat the end of the measurement tube. The detector and the parabolicmirror have the same optical axis; as a result of this, the central areaof the radiation, that is to be measured, is blocked off. In addition, amotor and a light box are required for the chopper. Moreover, mechanicaldevices with a separate drive are needed in order to prevent thepenetration of steam into the measurement channel of the infrareddetector, whereby a shutter has been arranged at the entrance to themeasurement tube that closes the measurement tube after eachmeasurement. In addition, a heating element is proposed in order to heatthe hollow mirror in a defined manner. In total, this sensor module isconstructed in a very expensive manner and is prone to malfunctioningand is cost-intensive as a result of the plurality of its components,especially its mechanically driven components.

A further infrared detector arrangement in a microwave oven is knownfrom DE-26 21 457 C3. In the case of this solution, the infraredradiation that originates from several locations of the article, that isbeing cooked, is registered sequentially. A pyroelectric detector isagain used in combination with a chopper arrangement that consists oftwo shutter diaphragms that rotate at different speeds, whereby theshutter diaphragms have several holes.

This detector arrangement does not have optical focussing components(lenses or mirrors) in order to restrict the visual angle. Instead ofthis, various locations of the cooking zone are sensed through the holesthat are applied to the second disk. The microwave oven is switched offwhen the highest temperature occurs.

This application is not suitable for small or lengthy objects, that areto be measured, since these do not cover a large portion of the cookingzone. In addition, further sources of error are possible as a result ofthe feature that, after several heating processes using a highertemperature, the surroundings of the cooking zone can be "hotter" thanthe article itself that is being cooked. Additional sources of error canarise via the inherent and reflected radiation from the rotating disks.

The detector arrangement is relatively expensive and is prone tomalfunctioning as a result, in particular, of the two moving parts thatare driven at different speeds.

SUMMARY OF THE INVENTION

The task that forms the basis of the present invention is to propose asensor module, especially for the contact-less measurement oftemperature via the measurement of infrared radiation, whereby themodule is usable over a large range of ambient temperatures with a highmeasurement accuracy, and possesses no moving parts and is usablewithout re-calibration upon exchanging it for another unit.

The basic idea of the present invention is to arrange a thermopile, anda temperature reference element in the immediate vicinity of oneanother, and to suitably pre-amplify the signals from the thermopile andthe temperature reference element, in each case, in order, subsequently,to derive a control signal or, as the case may be, a temperature signalfrom the difference in these signals.

A preferred usage possibility for the sensor module in accordance withthe invention resides in its use in domestic devices such as, forexample, microwave ovens, ovens for baking, etc. However, the use of thesensor module is not restricted to the area of domestic devices but alsocomprises, rather, every measurement of surface temperature withoutmaking contact with subject matter under observation, for example inovens and machines, in the industrial manufacturing area, and in processmonitoring or, as the case may be, process control.

The ability to exchange the sensor module without calibration of thedevice and a high measurement accuracy over a wide ambient temperaturerange of the module are achieved by the features:

that a temperature reference element with known characteristics(preferably a thermistor or PT100) is located in the immediate vicinityof the "cold" contacts of the thermopile, preferably in its housing;

that amplification by the pre-amplifier in order to pre-amplify thethermopile signal is set up in such a way in a calibration step prior toincorporation into the device that the permissible variations in thesensitivity of the thermopile of typically a few multiples of 10%, thatare usually present, are compensated;

that the transmission region of the second infrared filter is largerthan that of the first infrared filter at the thermopile sensor, as aresult of which the permissible variations in the transmission region ofthe second IR filter do not have any effect on the measurement accuracyafter exchange of the sensor module; and

that the characteristic temperature line of the compensated outputsignal from the sensor module is very precisely reproducible because ofpre-calibration of the amplification of the sensor and the knowncharacteristic line of the temperature reference element.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following sections, the invention will be elucidated in moredetail on the basis of the description of an example of an embodimentwith reference being made to the drawings. The following aspects areshown therein.

FIG. 1 shows a block circuit diagram of the sensor module in accordancewith the invention;

FIG. 2 shows the assembly of the sensor module in a microwave oven in aschematic, sectional form;

FIG. 3 shows the arrangement of the filter disk at the inner wall of amicrowave oven in accordance with a first form of embodiment;

FIG. 4 shows the attachment of a filter disk at the inner wall of amicrowave oven in accordance with a second form of embodiment; and

FIG. 5 shows a construction of filter disk with a metallic grid.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows the assembly of the sensor module inaccordance with the invention. One can see an object 1, that is to bemeasured, from which infrared radiation is fed into a first element 16of the sensor module via an infrared filter F2. Inside the element 16,the infrared radiation is focussed via a parabolic mirror 3 and isdeflected by 90°. A thermopile 6 is located at the focal point of thefocussed infrared beam. This is arranged in a housing 4 together with atemperature reference element 5. The infrared light beam enters thehousing through a filter F1 (7). The optical transmission region of thefilter F1, that lets..light through, is selected in such a way that itlies within the optical transmission region, that lets light through, ofthe filter F2, whereby the optical transmission region of the filter F2is somewhat larger than that of the filter F1. As a result, thesituation is reached in which, on exchanging the element 16, measurementerrors are not produced in regard to losing light intensity that isincident on the sensor because the filtering regions of the filters F1and F2 merely overlap partially and non-reproducibly.

The output from the thermopile 6 is connected to the input to apre-amplifier 9, whereby the pre-amplifier amplifies at an amplificationfactor that is capable of being calibrated and that is represented by acounter-coupling resistance 8. The temperature reference element 5--athermistor in this case--generates an input signal for the pre-amplifier12 that converts its output signal into a correction signal that isdependent on the ambient temperature. This takes place next to thethermistor 5 and the operation amplifier 12 via the resistances 10 and13 and the potential reference 11. As a result of the appropriate!selection of the resistance 10 and 11, the thermistor 5 and thepotential reference 11, one defines the temperature range in which asmall measurement error arises in the event of ambient temperaturevariations. In principle, other circuit variants are also possible forthe production of the correction signal. For the concrete assembly ofthe device, it is sufficient to stipulate, on one single occasion, thevalues of the thermistor 5, the resistances 10 and 13 and the potentialreference 11. They can thus be maintained in the case of large numbersof items. Variations in the sensitivity of the thermopile 6, includingthe filter characteristics of the first infrared filter 7, merely haveto be pre-calibrated by calibrating the amplification of thepre-amplifier 9 via the variable resistance 8. The calibration of theamplification can be achieved, for example, by means of laser trimmingof the resistances or by short circuiting or, as the case may be,selecting parallel or serial compensation resistances. The temperaturecompensation signal is drawn off into a difference amplifier 14 from theamplified thermocouple signal at the output of the first pre-amplifier.The output signal, that has been temperature-compensated in this way,from the difference amplifier 14 is fed into the electronic control unitECU 15 for further processing where, for example, it can be used forcontrolling or regulating baking processes, boiling processes or heatingprocesses. After calibration, each sensor module exhibits reproduciblesignal characteristics within certain tolerances, whereby the signalcharacteristics can be processed by the ECU 15. Measurements of thesurface temperature of a black body radiator of constant temperaturewith a sensor module in accordance with the invention have resulted inthe feature that the result of the measurement changes only by ±2° C.while the ambient temperature of the module was increased from 10° to90° C. Compensation of the total apparatus can thus be dispensed with ontaking it into service. Likewise, in the case of repair work, adefective module can be exchanged without calibrating the device. As aresult, a significant simplification of the servicing of the device ispossible. The response rate of the sensor is also very high so that arapid temperature change on the surface of the object, that is beingmeasured, can be registered in fractions of a second and incipientburning of the articles, that are being baked, can be prevented by theimmediate switching off of the device.

FIG. 2 shows a sensor module in accordance with the invention that hasbeen incorporated into a microwave oven. The element 16 is built intothe intermediate zone between the inner housing wall 21 and the outerwall 20 of the microwave oven device and can also be shielded off, ifrequired, from interference, that emanates from the microwave device, bymeans of a metallic screen 27. Infrared radiation from the object 22,that is to be heated, gets into the element 16 through the filter F2that is transparent to infrared and that is inserted in the intermediatewall 21. Thereby, the edges of the filter F2 are connected to the innerwall of the microwave oven in an electrically conducting manner. Thetemperature-compensated output signal from the element 16, thatcorresponds to the output signal from the difference amplifier 14, isled via an optionally shielded cable 29 to the ECU 15 control unit thatis arranged at a certain separation from the element 16 in the device20. There, the temperature-compensated output signal, that reproducesthe surface temperature of the article that is being cooked, can becompared with a target value after which, for example, switching off ofthe magnetron 23 of the microwave oven takes place. If the signal isread via analog/digital conversion into a processor, then the cookingprocess can be controlled in several stages (e.g. in cycles or via areduction in the magnetron output after reaching a certain surfacetemperature and switching off after reaching the final temperature).

Soiling of the second filter F2 is prevented by way of the feature thatpart of the stream of cooling air 24 from the magnetron cooling unit isled through a by-pass directly in front of the filter F2. FIG. 2 showsthe branched air stream 26. However, if the sensor filter becomes soiledafter extended or improper use (e.g. by splashes from the article thatis being cooked), then the filter disk F2 can readily be cleaned by theoperator himself/herself. Soiling of the parabolic mirror optical systemor of the detector input filter F1, that cannot be cleaned competentlyby the user, is in any case prevented. The opening angle 28 of theinfrared beam of rays, that is evaluated by the sensor module, can berestricted to a few degrees by the proposed parabolic mirror opticalsystem so that accurate localization results on the article that isbeing cooked. This is especially important since the amounts ofradiation that are detected from the surroundings can significantlyfalsify the result of the measurement. In addition, the small openingangle also permits the evaluation of small objects that are to bemeasured. If, during usage in the microwave oven, one starts outexclusively from objects, that are to be measured, that have a largesurface area, then the sensor module can also be arranged at the sidewalls of the microwave housing. In this case, the beam of rays is notdirected vertically but, rather, obliquely onto the article that isbeing cooked. The installation of the filter F2 in the inner wall 21 ofthe housing is described in more detail in FIGS. 3 and 4 that show twodifferent forms of embodiment. Thus, in accordance with the form ofembodiment that is shown in FIG. 3, the filter disk F2 is held in ametal frame 31 and is engaged in a recessed depression in the metal,inner wall by means of a connection 32 that permits clicking intoposition.

FIG. 4 shows the holding of the filter disk F2 by means of an adhesivebond 33 or by the application of a flexible strap 34. In order to reducethe flowing in of microwave radiation from the cooking zone through theopening 35 (FIG. 5) of the intermediate wall and into the sensor module,it is advantageous to manufacture the filter from an electricallyconductive material (e.g. Si or Ge) and to connect it to theintermediate wall in an electrically conducting manner. The adhesive 33must then be electrically conductive too or, as the case may be, it mustachieve stable contact by means of springy flexible straps. In addition,the flowing in of microwave radiation can also be achieved by a grid 36(FIG. 5), that comprises thin metallic conductive pathways, that isplaced on the filter and connect to the inner wall of the microwave ovenin an electrically conducing manner. These conductive pathways can beachieved inexpensively by means of an evaporation process on one largedisk for many filters at the same time. If the sensor module is used indevices where soiling of the parabolic mirror optical system or, as thecase may be, the sensor filter F1 is ruled out, then one can dispensewith the filter F2.

We claim:
 1. Sensor module comprising a concave mirror having a focalpoint, and a sensor element located at the focal point, the sensorelement producing an output signal, wherein the sensor element comprisesa thermopile and a temperature reference element located in theimmediate vicinity of the thermopile so that the thermopile and thetemperature reference element are illuminated by the same radiationreflected from the concave mirror, the thermopile and the temperaturereference element producing respective output signals, wherein theoutput signal of the sensor element is to be transformed into atemperature signal in an evaluation circuit; whereinthe sensor modulefurther comprises a first pre-amplifier, that is capable of beingcalibrated for amplifying the output signal from the thermopile; asecond pre-amplifier amplifying the output signal from the temperaturereference element; and a third pre-amplifier connected between outputterminals of the first and the second pre-amplifiers to serve as adifference amplifier to form a difference in signal between outputsignals of the first pre-amplifier and the second pre-amplifier, anoutput signal of the third pre-amplifier serving as the output signal ofthe sensor element.
 2. Sensor module in accordance with claim 1, furthercomprising a first infrared filter with a first optical transmissionband and located in a path of rays of radiation incident upon the sensorelement from an object under observation by the sensor module.
 3. Sensormodule in accordance with claim 2, further comprising a second infraredfilter located between the concave mirror and the object, wherein anoptical transmission band of the second infrared filter is larger thanthe optical transmission band of the first infrared filter.
 4. Sensormodule in accordance with claim 3 for use in a microwave oven energizedby a magnetron cooled by a main stream of air flow, the oven beingconstructed with an inner wall and an outer wall, the inner walldefining an inner zone of the oven, wherein the thermopile, the firstinfrared filter, the temperature reference element, the concave mirrorand the pre-amplifiers are located in an intermediate zone between theinner wall and the outer wall of the microwave oven, whereas the secondfilter is located in an opening of the inner wall, and wherein a partialstream of the air flow is diverted from the main stream, in the form ofa by-pass into the inner zone of the oven through an opening in theinner wall in front of the second infrared filter whereby the partialstream flows past the second infrared filter.
 5. Sensor module inaccordance with claim 4, wherein the second infrared filter consists ofan electrically conducting material and the edges of the filter areconnected to the inner wall of the microwave oven in an electricallyconductive manner.
 6. Sensor module in accordance with claim 5 whereinthe electrically conducting material of the second infrared filter issilicon.
 7. Sensor module in accordance with claim 4 wherein the secondinfrared filter is over-coated with a grid of thin metallic conductingpathways that are connected to the inner wall of the microwave oven inan electrically conducting manner.
 8. Sensor module in accordance withclaim 1, wherein the concave mirror is a parabolic mirror wherein raysof radiation from an object under observation are reflected from themirror to the focal point in a converging cone of rays having a solidangle of 90 degrees.
 9. Sensor module in accordance with claim 1,wherein the thermopile and the temperature reference element arearranged in a common housing of the sensor element.
 10. Sensor module inaccordance with claim 1, wherein the temperature reference element is athermistor.
 11. Sensor module in accordance with claim 1, wherein thefirst and the second pre-amplifiers are arranged together on a housingof the sensor element.